Data Relay Satellite Constellation

Spacesys has made extensive studies concerning a constellation of small satellites injected in Intermediate Circular Orbits (ICO) to set up a distributed Data Relay System (DRS) which is both flexible, performant and cost effective. Our proposed system intends to overcome some drawbacks of current Data Relay Systems, based on geostationary satellites, including the rather poor Relay Satellites visibility by ground terminals in proximity of the polar caps; the delays in relaying data acquired by observation satellites not in direct view of the Data Stations; the large Earth-to-DRS distance which impacts negatively the link characteristics, and the potential congestion problems in handling many observation satellites simultaneously accessing one geostationary satellite per continental area.

A ‘distributed’ Data Relay system was conceived which, suitably connected by intersatellite links (ISL) between co-orbiting satellites, achieves a nearly 100% coverage of the globe, a high operational flexibility, a high reliability and an increased multiple access capability thanks to the ‘distributed’ nature of the system’s resources.

The system benefits from advanced technologies, among which the use of millimeter waves for ISL, non-transparent repeaters, and data speed conversion, but moreover trades bandwidth with time: in other words exploits the large time intervals between image data takes to lower the datarates required to relay data to Earth.

The Study did consider the functional aspects of the associated Ground Segment, as well as the key launch and orbit injection strategies. The latter were tackled comparing several alternatives including the use of VEGA and electric propulsion (E.P.) for orbit raising.

System Concept and motivations

This Study was originated by the growing dual use data relay requirements which include:

- very short delays between the images takes and their retransmission to the processing centers. In view of the globalization of observation missions, short delays are also needed when the observation satellite is very far away from the Data Receiving Station(s);

- simultaneous access to multiple LEO satellites involved in surveillance missions;

- wide geographical coverage of satellites circling the Earth, even when overflying the polar caps;

- the capability of monitoring and commanding spacecraft manoeuvers in near realtime even when the satellites are not in direct view of the ground station(s);

The most demanding applications do include post-disaster and damage assessment, providing support during emergencies, and tactical data relay. Besides imaging, the relaying of data to Earth may target the characterization of man-made systems RF spectral emissions and, in general, physical observable phenomena; and might support secure intercontinental communications, even at network level.

The space segment configuration, resulting from exhaustive trade-offs, consists of two triplets of equispaced satellites, injected in circular orbits of 6 hours period. The orbit semiaxis allows an unobstructed line of sight between each satellite of the triplet and the other two in the same orbital plane to achieve the required interconnectivity. Noteworthy, the system does without cross links between satellites injected in different orbital planes, this simplifying considerably the ISL management. An ISL distance of about 29000 km results, with a constant depression angle of 30° w.r.t the satellite nadir, which implies the use of fixed pointing high directivity antennas to implement the ISL. The Study did consider circular orbit inclinations between 30° and 90° along with RAAN spacing between 45° and 180°. One good choice is.a pair of polar orbits with a relative RAAN spacing of 90°.

The connectivity results are shown below..

All six spacecraft , in stacks of three held by a support structure, would be brought by a single VEGA at 400 km altitude. From there, the spacecraft will reach their destination orbit using chemical propulsion. In alternative an electric-propulsion solution was also considered wherein a spiralling ascent to the final destination altitude would take place; however the manoeuver would require nearly one year which was felt to be quite unpractical.

Transmission system

The main objective is maximizing the data volume retransmitted to Earth while minimizing the time delay between data acquisition and delivery to the data processing center. The transmission system, based on non-transparent repeaters and a clever time-bandwidth exchange, uses the 26 GHz band for both ISL and links with the observing LEO satellites. We defined a convenient datarate range in a 20 to 60 Mbps and estimated the time required to transmit 1 Gbps of data. At 60 Mbps it takes 17 seconds which seemed a reasonable value, accordingly it was selected as a baseline transmission rate.

The feederlinks operate in the X_band, the choice being due to the current standards, but a Ka_band choice is certainly feasible.

The DRS multi-transponder includes:

- two Ka_band transponders handling two symmetrical Intersatellite links with the adjacent DRS. The transponders are each provided with a fixed-pointed Ka_band, 40 cm side planar array antenna, with a 2° beamwidth. The antennas are stowed during launch and in-orbit deployed at an angle of 30° w.r.t. the spacecraft nadir.

- a Ka_band transponder handling the interorbit link between the LEO observation satellite and the DRS spacecraft. The transponder is also equipped with a planar array antenna which is stowed during launch and in orbit deployed. Boresight repointing must cover a cone of 24° half angle, using a two-axis pointing mechanism put on one edge of the planar array;

- an X_band transponder handling the DRS to Earth links. This transponder is equipped with a mechanically steered square planar array antenna with a 40 cm side, and a beamwidth around 4.7°. For beam repointing the same mechanism adopted for the other antennas has been baselined;

- a baseband switching matrix interconnects the input/outputs from/to all transponder units. The matrix, which operates on the regenerated bit streams, supports the payload reconfiguration in the operating modes. It can be preprogrammed or remotely programmed from the Control Center knowing the LEO satellites future paths and their needs to transmit to Earth the gathered data

Link budgets have been performed for all links and show a positive margin..

The Ground Segment

This distributed Data Relay Satellite System has been conceived to remotely command a LEO spacecraft, receive telemetry data and retrieve without delays the data -mainly images- acquired by the observation instruments independently from the LEO spacecraft actual and past orbital position, using at least one station preferably located in the national territory . Obviously the distributed system nature is compatible with a multiplicity of other stations both located in the national territory and in other countries spread over the continents. The total data volume that can be handled daily by the six satellites of the constellation working continuously and simultaneously, and routing data at a nominal bitrate of 60 Mbps, is of the order of 31 Terabit.

Accordingly, a distributed DRS system based on minisatellites in ICO, may well serve the data relay needs of a multiplicity of countries, relying on the discontinuous operation of their observation satellites.

The architecture of each Ground Station does envisage the operational, independent, presence of two tracking X_band antennas having a minimum diameter of 2.5 m. Indeed, the constellation time evolution is such that two D.R. satellites , one per triplet, will always be in the field of view of ground stations positioned at latitudes greater than about 25°. Ground stations equipped with two independent antennas might therefore have simultaneous access to two DRS spacecraft in the same or different orbits.

The spacecraft design

The spacecraft design aims at implementing all functions within the typical size of a microsatellite with a launch mass lower than 150 Kg, see the mass budget in the Table herebelow.

Spacesys has performed several trade-offs among the candidate architectures. An all-electric propulsion solution is too complex from a solar plant viewpoint (see the figure below showing that to cope with electric propulsion requirements the satellite would have to implement multiple foldable /unfoldable solar panels impacting cost and reliability), hence a chemical-propulsion alternative seems a sounder approach. It is now planned to baseline a hydrazine propulsion subsystem, which would make easier the task of thermal control too.

Copyright: SpaceSys , Via Etruria 47, 00183 Rome, Italy; Phone: : +39 0664821022 ; P.IVA: 10034621002 ;

updated: April 2011